System for diagnosing an internal combustion engine

ABSTRACT

A system for diagnosing an internal combustion engine having a coolant temperature sensor. A coolant temperature diagnosing circuit is provided which is responsive to the output voltage of the sensor for producing a signal when the voltage exceeds a predetermined range. A fail-safe signal generating circuit is provided which is responsive to engine speed and to produce fail-safe signals dependent on the engine speed so as to prevent stalling of the engine.

BACKGROUND OF THE INVENTION

The present invention relates to a diagnosis system for internalcombustion engines, and more particularly to a system for diagnosingfailure of a coolant temperature sensor of an engine mounted on a motorvehicle.

An electronic fuel-injection system of the type which is provided with acomputer operable to compute various factors, such as throttle position,volume of intake airflow, intake manifold vacuum and coolant temperaturein order to produce output signals for driving solenoid-operatedinjection valves is known. A coolant temperature sensor is provided toproduce an output signal when the temperature rises to a predeterminedvalue. The computer uses the output signal as an input signalrepresenting engine warming-up. However, if a particular fault occurs,such as a disconnection of wires for the sensor or a short circuiting ofthe sensor, the sensor produces an output having an extreme value, whichresults in a failure in controlling the air-fuel ratio of injected fueland in stalling of the engine.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system fordiagnosing fault in a coolant temperature sensor system, in which aquasi temperature signal is produced in accordance with engine speed andthe quasi temperature signal is used as a coolant temperature signal soas to prevent failure of starting of an engine or stalling of theengine.

According to the present invention, there is provided a system fordiagnosing an internal combustion engine having a coolant temperaturesensor; a coolant temperature diagnosing circuit responsive to outputvoltage of said coolant temperature sensor for producing an output whensaid output voltage exceeds a predetermined range; means for generatingoutput voltage dependent on engine speed; a fail-safe generating circuitresponsive to said output voltage of said means for producing one offail-safe signals in dependency on said output voltage; and switch meansresponsive to said output of said coolant temperature diagnosing circuitfor allowing said fail-safe signal to pass.

The present invention will be more apparent from the followingdescription made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a diagnosis system for operation of anengine;

FIG. 2 is a graph showing a relationship between engine speed and quasisignals in the system of the present invention; and

FIG. 3 is a coolant temperature sensor diagnosing circuit according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 diagrammatically shows a diagnosis system for operation of anengine including a system of the present invention. The system comprisesan engine operation detecting means group 1 to 9, and a control unit CU.The control unit CU comprises a computor 11 including memories, I/O andtimers, an interface group 17a to 17g and, drivers 18a to 18c. Thedetecting means group comprises an ignition signal generating circuit 1,the output signal of which represents an ignited cylinder of the engineand the ignition timing thereof, an airflow meter 2, a coolanttemperature sensor 3, an intake air temperature sensor 4, and an O₂-sensor 5 for detecting the oxygen concentration in exhaust gases fromthe engine. The detecting means group further comprises a starter switch6 for detecting the operation of the starter of the engine, a vacuumswitch 7 for detecting the vacuum in the intake passage of the engine,an idling detecting switch 8 which is operated by a throttle valve shaftat the idling position of the throttle valve, and a full load switch 9which is also operated by the throttle valve shaft at a wide openthrottle position of the throttle valve. The computer 11 comprises aself-diagnosis function section 11a, a fail-safe signal generatingsection 11b, a fuel injection control signal generating section 11c anda display change signal generating section 11d. The self-diagnosisfunction section 11a is connected to a terminal 10 for checking fault.When the terminal 10 is connected to the ground at a shop, a faultcondition memorized in the display change signal generating section 11dis displayed by a lamp 16. Further, outputs of switches 6 to 9 areapplied to sections 11a and 11c through interfaces 17c to 17f,respectively.

The self-diagnosis function section 11a monitors inputs from the engineoperation detecting means group 1 to 9 and when any fault is detected, asignal is sent to a warning lamp 15 through the driver 18c to warn ofthe fault. Further, upon the occurrence of serious engine trouble whichwould stall the engine, the self-diagnosis function section 11a sends adiagnosis signal dependent on the kind of the engine trouble to thefail-safe signal generating section 11b. The fail-safe signal generatingsection 11b stores a plurality of data to avoid the stalling of theengine caused by the engine trouble and produces a fail-safe signaldependent on the diagnosis signal. The fail-safe signal is fed to thefuel injection control signal generating section 11c which operates soas to stop the input from the detecting means group 1 to 9.

In normal engine operation, the fuel injection control signal generatingsection 11c operates to produce an air-fuel ratio control signal bycomputing the inputs applied from the engine operation detecting meansgroup 1 to 9. The air-fuel ratio control signal is fed to a fuel pump 13through the driver 18b and a relay 14 and to fuel injector controlcircuits 12 through the driver 18a so as to inject a proper amount offuel at a proper time. Further, the fuel injection control signalgenerating section 11c sends a signal to the switching section 11d inresponse to an input from the O₂ -sensor 5. The switching section 11dsends a signal to a monitor lamp 16 in response to the signal from thefuel injection control signal generating section 11c to indicate thefact that the exhaust gases concentration is normal.

When the fail-safe signal is fed from the fail-safe signal generatingsection 11b to the fuel injector control signal generating section 11c,the section 11c produces a quasi air-fuel ratio control signal dependenton the fail-safe signal. The quasi air-fuel ratio control signal is sentto the fuel injection control circuits 12, so that the engine continuesto operate in accordance with the quasi signal without stalling.

Further, the switching section 11d sends a signal to the lamp 16 independency on the diagnosis signal fed from the self-diagnosis function11a. The lamp 16 intermittently lights in accordance with a patternwhich is decided by the diagnosis signal by connecting the terminal 10to the ground at a shop. An inspector in the shop can recognize the kindof the engine trouble by the pattern of the lighting of the lamp 16.

In accordance with the present invention, when fault occurs in thecoolant temperature sensor system, the self-diagnosis function section11a generates a signal and the signal is fed to the fail-safe signalgenerating section 11b.

Referring to FIG. 2, curve A shows engine speed, in which N₁ is anengine speed at cranking of an engine and N₂ is an engine speed atidling of the started engine. Curve B shows fail-safe values TF₁, TF₂and TF₃ dependent on the engine speed.

FIG. 3 shows a diagnosing circuit 21 for coolant temperature, which isincluded in the self-diagnosis function section 11a (FIG. 1), and afail-safe generating circuit 37 which is included in the fail-safesignal generating section 11b. The output of the coolant temperaturesensor 3 is fed to a window comparator comprising comparators 22 and 23and an OR gate 24, in the coolant temperature diagnosing circuit 21, andfurther fed to a computing circuit 38 through an analog switch 36. Theoutput of the window comparator is applied to an AND gate 26 and alsoapplied to an AND gate 27 through an inverter 28. AND gates 26 and 27are applied with pulses from an oscillator 25 to produce pulsesdependent on the outputs of the window comparator.

The coolant temperature diagnosing circuit 21 is provided with a firstcounter 29 for detecting the coolant temperature and a second counter 32for resetting the first counter 29.

If the output voltage of the coolant temperature sensor exceeds apredetermined range of the window comparator, the output of the OR gate24 becomes high, so that the AND gate 26 produces pulses. The pulses areapplied to the counter 29 through an AND gate 31. When the count of thecounter 29 exceeds a set count upon a fault of the sensor, the counter29 produces a high level output. The output is applied to the otherinput of the AND gate 31 through an inverter 30, so that the AND gate 31is closed to hold the output of the counter 29. The output of thecounter 29 is applied a control gate of an analog switch 34 to close itand also applied to the control gate of the analog switch 36 through aninverter 35 to open the switch.

It is necessary to reset the counter 29, if the trouble in the coolanttemperature sensor recovers after a short time. The counter 32 is toreset the counter 29. The output of the OR gate 24 is at a low level innormal operating conditions, so that the AND gate 27 is opened toproduce pulses. When the number of pulses applied to the counter 32reaches a predetermined value which corresponds to the above describedshort time, the counter 32 produces an output. The output is applied toa reset terminal of the counter 29 to reset it and also applied to itsown reset terminal through a delay circuit 33. Thus, the reset operationof the counters 29 and 32 is repeated a long as the coolant temperaturesensor 3 is in normal condition. When a fault occurs in the coolanttemperature sensor and the analog switch 34 is closed by the output ofthe counter 29, a fail-safe signal from the fail-safe signal generatingcircuit 37 is fed to the computing circuit 38 through the switch 34. Thecomputing circuit 38 produces a quasi air-fuel ratio control signal inresponse to the fail-safe signal. The quasi air-fuel ratio controlsignal is fed to a fuel injector 40 through a driver 39 to inject thefuel at a proper air-fuel ratio so as to keep the engine operation. Inaddition, the output of the counter 29 is fed to the lamp 15 for warningof the fault.

The fail-safe signal generating circuit 37 is adapted to produce threekinds of fail-safe signals in dependency on engine speed. The enginespeed signal is produced by a waveform shaping circuit 41 for shapingignition pulses from the ignition signal generating circuit 1 and by afrequency-to-voltage (F/V) converter 42 and is applied to a windowcomparator comprising comparators 43 and 44 and a NOR gate 45. Theoutputs of the comparators 43 and 44 are applied to bases of transistorsTr₁ and Tr₂, respectively, and the output of the NOR gate 45 is appliedto a base of transistor Tr₃.

When engine speed exceeds the engine speed N₂, the comparator 43produces a high level output voltage which turns on the transistor Tr₁.When engine speed decreases below the engine speed N₁, the comparator 44produces a high level output voltage, so that the transistor Tr₂ isturned on.

Between N₁ and N₂, the outputs of comparators 43 and 44 are at lowlevels, so that the output of the NOR gate 45 becomes high, turning onthe transistor Tr₃. Thus, one of three voltages decided by resistors R₁,R₂ and R₃ is applied to the computing circuit 38 as a fail-safe signalin dependency on the engine speed so as to prevent stalling of theengine.

While the presently referred embodiment of the present invention hasbeen shown and described, it is to be understood that this disclosure isfor the purpose of illustration and that various charges andmodifications may be made without departing from the spirit and scope ofthe invention as set forth in the appended claims.

What is claimed is:
 1. In a system for diagnosing an internal combustionengine having a coolant temperature sensor, the improvement comprisingacoolant temperature diagnosing circuit responsive to output voltage ofsaid coolant temperature sensor for producing an output when said outputvoltage exceeds a predetermined range; means for generating an outputvoltage dependent on engine speed; a fail-safe signal generating circuitresponsive to said output voltage of said means for producing one offail-safe signals in dependency on said output voltage of said means;and switch means responsive to said output of said coolant temperaturediagnosing circuit for allowing said fail-safe signal to pass.
 2. Thesystem for diagnosing an internal combustion engine according to claim 2wherein said coolant temperature diagnosing circuit comprises acomparator, means for changing the output of said comparator to pulses,and a counter for counting said pulses, said counter being adapted toproduce an output when the count reaches a predetermined count.